6,787 MILES OF GAS LINE-AND IT LEAKS

By JAMES BARRON

Published: April 12, 1987

FOR two days last week, customers in a South Bronx grocery store smelled gas. On Wednesday, the second day, the distinctive odor was so strong the manager sprayed air freshener around the checkout counter in the afternoon and told a clerk to call Con Edison.

By then it was too late. Within 20 minutes, while a Con Edison crew was hurrying to check the report, a gas explosion demolished the store and leveled an apartment building next door, killing six people and injuring 30 others.

The apparent origin of that gas leak was unusual - bullets had struck the gas meter in the store's basement - but gas leaks are not. Con Edison logged 58,000 reports of gas leaks last year, and utility experts fear that far more people never bother to report gas odors.

''Too many people just say, 'Gee, I smell something,' '' said Richard M. Kessel, the executive director of the state Consumer Protection Board.

No one knows how many injuries or deaths are caused by gas leaks, and gas is not always identified as a culprit. When an explosion in an underground vault for electrical equipment blew off a manhole cover on Manhattan's Upper West Side last month, injuring nine schoolchildren on a field trip, Con Edison at first said there was no evidence of a gas leak.

But last week, after studying air samples, the utility acknowledged that gas had seeped from a pipe, setting off a blast so powerful that it ripped off the door of a car parked nearby.

Con Edison has some 6,787 miles of gas mains to maintain. Some mains date to World War II, though the utility - like most around the nation -has been replacing its old cast-iron pipes with stainless steel or plastic tubing.

And as Terry Uhl, a spokesman for the American Gas Association, an industry group, observed, the chance of a gas leak setting off an explosion is greater in cities than in rural areas. ''In an urban area,'' Mr. Uhl said, ''there are so many more meters and so many more lines that the numbers make something more likely to happen.''

Utility companies say, however, that the infrastructure is not the biggest source of gas leaks. Far more common, and more troublesome, are gas leaks caused by customers who fiddle with the meters in their homes. Sometimes consumers whose gas service has been shut off will try to turn it back on. Sometimes customers install furnaces or ranges themselves instead of calling in a professional.

Most of the time, natural gas is harmless. In the old days, when gas was made from coal, it gave off potentially lethal carbon monoxide. But the gas now in use is a non-toxic petroleum product; a person may die in a gas-filled room, but the cause of death will be suffocation, because the gas has squeezed out the air -not the gas itself.

To ignite, gas must be mixed with the right amount of air, roughly one part gas to nine parts air. Relatively little gas escapes from a stove when its pilot light goes out, utility officials say, so that striking a match to relight it is generally safe. Furnaces use larger amounts of gas, but most have heat-sensitive switches that act as stopcocks, blocking the flow of gas when the pilot goes out. When to Beware

Danger arises when a leak builds in a poorly ventilated space, as was probably the case in the basement of the South Bronx grocery last week. In such circumstances, a tiny spark can set off a conflagration. That is why utilities advise consumers who smell gas at home to leave and call the utility from somewhere else. Just picking up a telephone receiver can cause electrical activity that, if the gas concentration is high enough, could lead to disaster.

Until World War II, natural gas was an unused byproduct of oil refining. Distributors soon realized its potential and began laying pipes, however, and the industry grew as suburbia grew.

Last week, as fire officials sifted through the rubble of the grocery and investigators began trying to find out who fired the bullets at the gas meter, consumer advocates were suggesting changes. ''One of the things we ought to look at is a kind of a 911 emergency system for areas that are serviced by gas,'' Mr. Kessel said.

Con Edison enters gas-leak reports on a computer, but the calls are not recorded on audio tape, as 911 calls are. Some of the grocery's customers recalled the owner's saying that he had told someone to call Con Edison on Tuesday, the day before the explosion. Martin Gitten, a spokesman for the utility, said the company could find no record of such a call.

Leaks on the customer's side of the meter are generally the customer's responsibility, although Mr. Gitten said Con Edison's crews will turn a screw or twist a bolt if that is all that is necessary. If the leak is serious enough, the utility is required to shut off service and place red warning tags on the affected appliances.

In New York City, he said, service cannot be reinstated until the red tag is removed - and the tag cannot be removed until a licensed plumber has completed the repair and a city inspector has inspected it.

Of more concern, Mr. Kessel said, is how fast utilities must respond to gas-leak calls.

''Con Ed may have done the best job it could under the circumstances,'' Mr. Kessel said. ''But now that the tragedy has occurred, the state needs to establish standards.''

Mr. Gitten said Con Edison is usually on the scene within an hour. But on Wednesday, that was not fast enough.

Photo of rescue workers searching for survivors in the South Bronx (NYT/Ruby Washington)